1. Field of the Invention
The present invention relates to the structure of a frequency stabilized laser with mode hopping suppressed in an external cavity type frequency stabilized laser composed of an optically induced grating in an optical waveguide and a semiconductor LD (laser diode).
2. Description of the Related Art
A laser composed of an optically induced grating in a silica glass waveguide and a semiconductor LD is expected to find various uses as light sources for optical communication, optical information processing, optical measurement, and spectroscopy because of the following features: (1) It performs single-mode oscillation by utilizing the frequency selectivity of the grating. (2) Its temperature coefficient is smaller than that of a semiconductor laser. (3) Its oscillation frequencies can be controlled easily. (T. Tanaka, et al., Electron. Lett., vol. 32, No. 13, 1202 (1996); and Tanaka, et al., Presentations at the 1997 Congress of the Institute of Electronics, Information and Communication Engineers, C-3-160). The technique for fabrication of an optically induced grating was invented by Kenneth O. Hill, et al. (Japanese Patent Application Laying-open No. 7-140311(1995)). The optically induced grating will be referred to hereinbelow as a grating for simplification.
FIG. 13 is a schematic perspective view of a frequency stabilized laser produced by an earlier technology. In FIG. 13, the reference numeral 11 designates a semiconductor LD, 13 a core of a silica-based waveguide, and 14 a cladding of the silica-based waveguide. The reference numeral 15 denotes a grating, 16 an Si substrate, and 18 a portion, called a silicon terrace, which has been formed by removing silica glass for installation of the semiconductor LD.
The oscillation modes of the frequency stabilized laser composed of the grating in the silica-based waveguide and the semiconductor LD will be described. When an injection current is flowed into the semiconductor LD to cause light emission, only light of frequencies corresponding to the reflection spectrum of the grating is reflected by the grating. Thus, oscillation originates from a laser cavity which is a zone from the rear facet of the semiconductor LD to the grating. On the output endface of the semiconductor LD, an antireflection film to the interface with air is provided so that there will be no external feedback to the semiconductor LD except from the grating and the rear facet of the semiconductor LD. Also, the LD-side end face of the silica-based waveguide has a core-adjoining portion inclined with respect to a direction perpendicular to the optical axis of the core (see Japanese Patent Application Laying-open No. 6-230237 (1994)). Generally, the bandwidth of the reflection frequencies of a grating is about 50 GHz. On the other hand, the length of a laser cavity is about 0.5 cm. Thus, the frequency spacing of longitudinal modes is about 20 GHz. Since longitudinal modes exist with 20 GHz spacing in the 50 GHz bandwidth, about 3. longitudinal modes can be present. Of these longitudinal modes, only the one closest to the reflection center frequency of the grating is selected for oscillation. Generally, the reflectance of the grating is 40 to 99%, and the optical coupling loss between the semiconductor LD and the silica-based waveguide is about 4 dB.+-.1.5 dB.
With the conventional frequency stabilized laser, however, the frequency of the longitudinal mode selected depends on temperature, causing a phenomenon in which the oscillating mode changes with a temperature change (to be called mode hopping). The reason will be explained as follows:
The temperature coefficient of the longitudinal modes of a conventional frequency stabilized laser is expressed in an approximated manner by the equation (1) ##EQU1##
where
m.sub.LD and m.sub.WG are the temperature coefficient of the resonance frequencies of a resonator of a semiconductor LD, and the temperature coefficient of the resonance frequencies of a cavity prepared from a silica-based waveguide, respectively, PA1 n.sub.LD and n.sub.WG are the equivalent index of a guide layer of the semiconductor LD, and the equivalent index of the silica-based waveguide, respectively, and PA1 L.sub.LD and L.sub.WG are the cavity length of the semiconductor LD, and the silica-based waveguide length from the exit end of the semiconductor LD to the center of the grating, respectively. The grating is written into the silica-based waveguide, and the temperature coefficient of the reflection center frequency is equal to the temperature coefficient m.sub.WG of the silica-based waveguide. Since m.sub.LD.apprxeq.10 m.sub.WG, the magnitude of the temperature coefficient m of the longitudinal modes is larger than the temperature coefficient m.sub.WG of the reflection center frequency of the grating.
That is, the temperature coefficient of the longitudinal modes does not equal the temperature coefficient of the reflection center frequency of the grating.
FIG. 14 is an explanatory drawing for mode hopping. Assume that oscillation is occurring in the Nth longitudinal mode. As a result of a change in temperature, the longitudinal mode closest to the reflection center frequency of the grating shifts to the (N+1)th, where oscillation takes place. That is, mode hopping occurs. In an example described in the paper T. Tanaka, et al., Electron. Lett., Vol. 32, No. 13, 1202 (1996), mode hopping occurred each time the temperature changed by 5.degree. C. Since mode hopping increases the error rate of transmitted signals, realization of a method for suppressing it was desired.
The present invention has been accomplished to solve the above-described problem. Its object is to provide a frequency stabilized laser whose mode hopping due to a temperature change is suppressed by matching the temperature coefficient of longitudinal modes to the temperature coefficient of the reflection center frequency of a grating.